1. Field of the Invention
The present invention relates to an automatic gain control device used for a sound CODEC (CODEC is a compound word of a coder and a decoder). More particularly, the present invention relates to an automatic sound gain control device for automatically adjusting gains of sound signals input from a sound input device, such as a microphone, to proper values, and a sound recording/reproducing device provided with such an automatic sound gain control device.
2. Description of the Prior Art
The sound CODEC is a device which has two functions of coding sound which is an analog signal and decoding the coded data. Using such a sound CODEC, a conventional sound recording/reproducing device converts analog sound signals into coded data and records the coded data on a recording medium such as a tape cartridge.
Some of the telephones having the answering function employ a sound recording/reproducing device using a semiconductor memory instead of a magnetic tape. In such a sound recording/reproducing device, data coded from sound signals are stored in the memory and then decoded to be reproduced as sound.
In order to improve the quality of sound reproduced, a gain of a sound signal must be adjusted to a proper value by a sound gain control device so that the amplitude of the sound signal can be corrected to a proper size. Conventionally, this gain control has been accomplished by a feedback control in which an averaged value of sound amplitudes or sound power obtained by monitoring sound signals is fed back as a gain coefficient. One method of sampling the characteristics of the sound signals is based on a frequency distribution obtained by a statistical technique.
However, when the sound signals are monitored for a long period of time, the frequency distribution is not uniform, with the sound signals of smaller amplitudes having a larger incidence probability and the sound signals of larger amplitudes having a smaller incidence probability. Therefore, the frequency distribution method fails to seize the characteristics of the sound precisely and thus to provide a precise amplitude coefficient. As a result, the sound quality is deteriorated. Especially, when a short speech is included during that long period of time, the sound can not be specified and thus information of a proper amplitude can not be obtained, thereby causing a serious problem. In addition, in the case of a speech, there is another problem of being affected by the intonation of the speech.
When the gain control is performed by successively feeding back to adjust a gain depending on the size of a sound signal, the sound obtained will lack in intonation and be monotonous. However, in a sound recording/reproducing device employed in the telephone with the answering function, which has a time lag between the recording of sound signals and the reproduction thereof, it is possible to determine how to adjust the gains afterwards. Making the best use of this advantage, a sound gain control device has been developed in which sound signals are sampled in each prescribed period, and an adjusting value of gain is calculated based on the average of amplitudes of the sound signals in each prescribed period and is recorded together with digital data of the sound signals. In this way, by performing the gain control of sound signals in each period, the intonation and other characteristics of the sound can be preserved.
A usual speech includes a sounding portion where a speech is generated and a silent portion between the punctuations of the speech. In a speech having many silent portions, the rate of the silent portions occupying the period is increased, whereby the average of the amplitudes of the sound signals is abnormally lowered. Therefore, in the conventional sound gain control device, when the gain is adjusted in a block of sound signals in each prescribed period, a proper gain adjustment for the sound having many silent portions is not possible. The period may be shortened, but this will produce the sound with poor intonation as in the case of the above-described successive feedback method, and will not solve the problem.
Further, in the conventional sound recording/reproducing device, since the sound signals are adjusted with the gains and coded before they are stored in a recording medium, the sound signals after the gain adjustment are decoded for reproduction. For this reason, in the conventional sound recording/reproducing device, the scale and intonation of the reproduced sound become monotonous, and therefore, the nuance of the recorded message can not be conveyed precisely.